buy
When you buy something, you get it by paying money for it. The past tense and -ed participle of buy is bought.
I'm going to buy everything that I need today.
He bought a first-class ticket.
If you pay for a drink for someone else, you say that you buy them a drink.
Let me buy you a drink.
Be careful
Don't say ‘Let me pay you a drink’.
Don't say ‘Let me pay you a drink’.